Centrifugal pumps of the above-mentioned type are usually present in the hitherto commercially available dishwashers.
They are designed to allow drainage of the dirty water used in the washing cycles of the electric household appliance into a waste water collection network.
For this purpose, such pumps use a bladed impeller driven by a small-size electric motor; the rotary motion of the impeller thus moves a stream of dirty water coming from an intake channel arranged along the axis of rotation of the part towards a delivery channel which is radial and tangential thereto.
However, when designing the impeller, it is necessary to take account of the specific function of the pump of which it forms part. This is because discharge pumps, unlike water recirculation pumps, are located downstream of a relatively coarse filter and the stream of water passing through them may become contaminated with small-size solids capable of stopping the device from functioning. Possible contaminants may consist of food residues, objects mistakenly introduced into the machine, such as toothpicks, or also suspensions originating from chemical interactions between the detergent and fatty substances present on the dishes.
In order to allow easy removal of the above-mentioned solids, a recessed impeller—i.e. an impeller whose blades do not extend sufficiently far to achieve a close clearance with the front wall of the working chamber—with straight blades is used. There are usually four such blades which are arranged at right angles to each other and extend radially from a central shaft connected to the motor.
The above-described geometry significantly reduces the risk of a pump blockage due to any foreign solids, since the latter have plenty of space to pass through, this being defined both by the interspace between impeller and front wall and by the volume present between adjacent blades.
While the discharge pumps according to the prior art are advantageous in various respects and are substantially fit for the purpose, they do have one major drawback which hitherto has not been overcome.
While, on the one hand, the above-mentioned geometry of the impeller does indeed ensure trouble-free operation even when solids are present in the discharge liquid, on the other hand it results in a level of hydraulic efficiency of the pump which is low compared to that which would be achieved if a close-clearance impeller with curved directional blades were to be used.
The limited hydraulic efficiency results in the need to use relatively large and powerful electric motors, resulting in greater wear of active materials such as copper and a consequent increasing in the cost of the discharge pump.
The technical problem underlying the present invention is consequently that of devising a discharge pump which allows removal, without any risk of blockage or jamming, of any solids which may contaminate the discharge water, but without this feature limiting the hydraulic efficiency of the pump itself.